Well, often they are on standby and are not confirmed into a seat or cabin until very close to departure.

There's no way to know if they would get a seat in Club Europe, so what happens if they don't and have been in the lounge?

The system as it is works well. I used to have to guest my ex in to the lounge as I had status and he was staff, so on the occasions when we'd travel together on his staff travel, he got in due to my status, but otherwise was ineligible.

It makes sense really.